1. Field of the Invention
The present invention relates to a method and an electronic control system to compensate for the expecting aging-related brightness loss of an electroluminescent element operated using AC voltage.
Electroluminescence technology has increasingly gained in significance in recent time. It allows the implementation of almost arbitrarily large homogeneous illuminated areas which are free of screens and shadows. At the same time, power consumption and overall depth are extremely low (in the magnitude of one millimeter and less). The typical applications include, in addition to the background illumination of liquid crystal displays, the backlighting of transparent films which are provided with inscriptions and/or graphics.
Electroluminescence (in short: EL) is understood as the direct luminescence excitation of luminescent pigments and/or luminophores by an electric alternating field. Electroluminescence elements (in short: EL elements) based on thick-film technology using inorganic luminescent pigments and/or luminophores and AC voltage excitation have become widespread. In relation to thin-film EL elements, thick-layer EL elements are less complex and thus more cost-effective to produce.
The luminescent pigments and/or luminophores are embedded in an organic or inorganic binder, which is as transparent as possible (or at least not completely opaque). Starting materials are usually zinc sulfides, which generate different, relatively narrow band emission spectra as a function of the doping and/or co-doping and preparation procedure. The focus of the spectrum determines the particular color of the emitted light.
The exciting AC voltage field typically has a frequency of a few hundred hertz, the effective value of the operating voltage frequently being in a range from approximately 50 to 150 volts. By elevating the voltage, a higher light density may typically be achieved, which is usually in a range from approximately 10 to approximately 200 candelas per square meter. A frequency increase usually causes a color shift toward lower wavelengths. However, both parameters must be tailored to one another to achieve a desired light impression.
2. The Prior Art
The service life of an EL element is limited. It is a function above all of the level and frequency of the AC voltage applied, in addition, however, also of environmental influences, particularly the effects of moisture and UV radiation. The service life of an EL element is typically specified as a half-life of the luminescent pigments. This is the time after which the light density has fallen to half of the starting value under the influences of the electrical field with unchanged operating conditions. In practice, the light density decreases to half of the original value within approximately 2000 to 3000 operating hours.
The reduction of the brightness because of the appearance of aging does not occur suddenly, but rather essentially continuously with increasing operating life.
A method and an electronic control system to compensate for the aging-related brightness loss of an electroluminescent element operated using AC voltage are known from U.S. Pat. No. 5,493,183. The operating life is cumulatively detected and the operating voltage is raised with increasing operating life to compensate for the aging-related brightness loss. The controller makes use of a table of stored comparative data, which shows the expected brightness reduction as a function of the operating life.
When the method known from U.S. Pat. No. 5,493,183 is applied, it is to be observed, inter alia, that the compensation of the aging-related brightness loss only succeeds inadequately depending on the mode of operation of a correspondingly controlled electroluminescent element. This problem occurs in particular if the voltage is also varied in another way in addition to counteracting the aging-related brightness loss, for example, to adapt the brightness of the EL element to the ambient brightness via voltage regulation.
An optical sensor which measures the brightness of the EL element could be used as a measure for more exact correction of the voltage to compensate for the aging-related brightness loss, but, on one hand, this would be connected with significant technical outlay and corresponding costs. On the other hand, a sensor of this type could be influenced by light from the surroundings and register a lower aging-related brightness loss than is actually present as a result.
In view of the above-mentioned problems in counteracting aging-related brightness loss according to the prior art, the present invention is based on the object of providing a method and an electronic control system to compensate for the expected aging-related brightness loss of an electroluminescent element operated using AC voltage, which functions reliably even if the electroluminescent element is operated at changing voltage and frequency levels. Furthermore, as much as possible, the method or the electronic control system is to be connected to no or only low additional costs.